This invention relates generally to plastic articles, and more particularly to polycarbonate articles exhibiting improved abrasion resistance, together with improved resistance to cracking under exposure to thermal and mechanical stresses.
Engineering resins are well-known, commercially available materials possessing physical and chemical properties which are useful in a wide variety of applications. For example, polycarbonates, because of their excellent breakage resistance, have replaced glass in many products, such as automobile headlamps and stoplight lenses; safety shields in windows, architectural glazing, and the like. However, major defects exhibited by polycarbonates are their very low scratch-resistance and their susceptibility to ultraviolet light-induced degradation.
Methods for improving the scratch-resistance of plastics such as polycarbonate have involved disposing an inorganic protective layer on the surface of the polycarbonate. For example, in U.S. Pat. No. 4,328,646, issued to Kaganowicz, an abrasion-resistant article is formed by subjecting a mixture of hardcoating precursors to a glow discharge, and depositing the product directly on a plastic substrate as a very thin film. However, inorganic hardcoatings such as silicon dioxide (SiO.sub.2) deposited directly onto plastics such as polycarbonate have performance problems when the system is subjected to stresses produced by mechanical or thermal effects. These problems are due to the difference in property characteristics of inorganic and plastic materials. For example, the thermal expansion coefficient for polycarbonate is about 7.times.10.sup.-5 m/m/.degree. C., while the coefficient for Pyrex.RTM. glass is 3.times.10.sup.-6 m/m/.degree. C. These differences result in tangential stresses at the interface of the plastic and the hardcoating which may in turn cause cracking of the hardcoating as a stress relief mechanism, especially when the article is subjected to various heating/cooling cycles. In attempting to alleviate this problem, Hall et al. and Geffcken et al. in U.S. Pat. Nos. 4,190,681 and 3,713,869, respectively, proposed the use of an intermediate layer to improve adhesion between the hard inorganic layer and the plastic layer. Furthermore, in Attorney Docket RD-16563 (Ser. No. 034,890), filed Apr. 6, 1987 for Devins et al. and assigned to the assignee of the present invention, there is disclosed an intermediate layer which not only improves adhesion but also provides a transition in properties from the plastic to the inorganic hardcoating, thus resulting in improved mechanical and thermal performance.
There is a continuing interest in improving methods for forming articles having still greater abrasion resistance while also exhibiting improvements in various other physical properties.
It is therefore an object of the present invention to provide a method for forming plastic articles having a high level of abrasion resistance, with improved resistance to cracking under exposure to thermal and mechanical stresses.
It is another object of the present invention to provide an improved method for applying smooth, hard, transparent layers over a thermoplastic substrate.
It is yet another object of the present invention to provide a thermoplastic article having disposed thereover a protective top layer characterized by uniform thickness, high abrasion resistance, and freedom from pinholes and microcracks.